In the past, RC networks and Schottky diodes have been used together in filter circuits. Such filter circuits are typically used to reduce electromagnetic and radio frequency interference (EMI/RFI) produced by computers and computer accessories.
These computers and accessories are routinely operated only a short distance away from telecommunication equipment. For example, a personal computer is often operated in close proximity to a television set and computers in police cars are operated side by side with police communication gear. Many of these computers have high clock rates and digital signals whose rise and fall times are extremely short. In addition, many computer accessories operate at similarly high speeds. For example, memory cards, video accelerator cards, modems, fax boards, co-processor cards, and data compression hardware all operate at very high frequencies. As a result, the high speed data lines in these computers and accessories produce EMI/RFI.
Without additional counter measures, these popular and widespread computers and accessories would contaminate the air waves and interfere with communication equipment. For example, cordless telephones, cellular telephones, radios, televisions, and burglar alarms could all be affected. Furthermore, important non-communication equipment, like pacemakers or other medical equipment, could also be impacted.
Therefore, it is desirable to have counter measures which suppress the EMI/RFI produced by the high speed data lines in computers and computer accessories. However, it is also desirable that such counter measures are inexpensive, occupy little space, ensure proper testing, and produce only small delays.
In the past, several techniques have been used for shielding high speed data lines. However, these methods have not proven to be effective at optimizing cost, size, ease of testing, minimization of delays, and suppression of EMI/RFI.
One method has utilized metallic shielding with feed through capacitors coupled to the high speed data lines. However, this method is expensive. Furthermore, it is no longer feasible in light of the need for portable computers and computer accessories and the availability of light weight plastic enclosures.
Another method has been to simply use low pass integrated RC filters produced by thin film techniques. The use of a resistor in series with a capacitor limits the maximum attenuation at higher frequencies to a constant value. However, such filters do not effectively suppress EMF/RFI produced by signal ringing on a transmission line.
Signals generally travel from one side of a transmission line to the other side. If the circuitry at the other end does not match the characteristic impedance of the transmission line, some of the signal amplitude can be reflected. This reflected signal can have the same or opposite polarity, depending on whether the mismatch is because of lower or higher impedance. As a result, signal ringing with undershoots or overshoots will occur. This signal ringing contributes to the radiation of EMI/RFI.
Negative undershoots are more likely to occur than positive overshoots. This is due to the fact that, in semiconductor technology, n-channel MOS transistors and NPN bipolar transistors are faster, more powerful, and therefore more commonly used than p-channel MOS transistors and PNP bipolar transistors.
To suppress signal ringing with undershoots, Schottky diodes can be connected in parallel with a low pass RC filter. The cathode of the diode is connected to the transmission line and the anode is connected to ground. For normal signal levels on the transmission line, the Schottky diode is reverse biased and does not conduct current. However, negative undershoots exceeding the forward diode knee voltage are clamped and limited in amplitude.
A Schottky diode is more effective than a regular junction diode at suppressing negative undershoots for two reasons. First, it has a lower knee voltage and therefore limits undershoots to a smaller amplitude. In other words, it suppresses ringing to a greater extent than does a junction diode. And second, unlike the junction diode, the Schottky diode has no carrier storage and therefore provides for a clean signal clipping of high frequency ringing.
In the past, Schottky diodes have only been used as discrete circuit elements in these filter circuits. Because of this, these filter circuits are costly to produce, occupy a great deal of space, require extra connections for testing, and have significant delays due to large spacing between elements.
Although Schottky diodes have been used as collector-base clamping diodes in Schottky-TTL technology, to date, they have not been integrated with RC networks on a single integrated circuit chip. This stems from the difficulty in producing such a chip by combining the techniques used for forming RC networks with the techniques used for forming Schottky diodes.